github.com/anacrolix/torrent@v1.61.0/metainfo/piece-length.go (about)

     1  // From https://github.com/jackpal/Taipei-Torrent
     2  
     3  // Copyright (c) 2010 Jack Palevich. All rights reserved.
     4  //
     5  // Redistribution and use in source and binary forms, with or without
     6  // modification, are permitted provided that the following conditions are
     7  // met:
     8  //
     9  //    * Redistributions of source code must retain the above copyright
    10  // notice, this list of conditions and the following disclaimer.
    11  //    * Redistributions in binary form must reproduce the above
    12  // copyright notice, this list of conditions and the following disclaimer
    13  // in the documentation and/or other materials provided with the
    14  // distribution.
    15  //    * Neither the name of Google Inc. nor the names of its
    16  // contributors may be used to endorse or promote products derived from
    17  // this software without specific prior written permission.
    18  //
    19  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    20  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    21  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
    22  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
    23  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
    24  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
    25  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
    26  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
    27  // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
    28  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
    29  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
    30  
    31  package metainfo
    32  
    33  // For more context on why these numbers, see http://wiki.vuze.com/w/Torrent_Piece_Size
    34  const (
    35  	minimumPieceLength   = 16 * 1024
    36  	targetPieceCountLog2 = 10
    37  	targetPieceCountMin  = 1 << targetPieceCountLog2
    38  )
    39  
    40  // Target piece count should be < targetPieceCountMax
    41  const targetPieceCountMax = targetPieceCountMin << 1
    42  
    43  // Choose a good piecelength.
    44  func ChoosePieceLength(totalLength int64) (pieceLength int64) {
    45  	// Must be a power of 2.
    46  	// Must be a multiple of 16KB
    47  	// Prefer to provide around 1024..2048 pieces.
    48  	pieceLength = minimumPieceLength
    49  	pieces := totalLength / pieceLength
    50  	for pieces >= targetPieceCountMax {
    51  		pieceLength <<= 1
    52  		pieces >>= 1
    53  	}
    54  	return
    55  }